First detection of Rumenfilaria andersoni in semi-domesticated reindeer (Rangifer tarandus tarandus) in Norway

Abstract Filarioid nematodes, including Rumenfilaria andersoni and Setaria tundra , are vector-borne parasites of cervids in Fennoscandia. We screened blood samples collected December 2024 – February 2025 from three herds of semi-domesticated reindeer ( Rangifer tarandus tarandus ) in middle and northern Norway (Trøndelag; n = 27, Nordland; n = 16; Finnmark; n = 28). Samples were examined for microfilariae using a Modified Knott’s Assay, followed by PCR and Sanger sequencing of a partial mitochondrial COX1 gene for species confirmation. Microfilariae morphologically consistent with R. andersoni were detected in three animals from Finnmark ( n = 3/28, 10.7%, [95% confidence interval (CI) 3.7–27.2]) with a mean intensity of 353 microfilaria per mL (mf/mL, range 63–881). There was no evidence of S. tundra microfilariae in the same animals ( n = 0/28, 0% [95% CI 0.0–12.1]). No microfilariae were detected in the animals from Trøndelag ( n = 0/27, 0% [95% CI 0.0–12.5]) or Nordland ( n = 0/16, 0% [95% CI 0.0–19.4]). COX1 sequencing confirmed R. andersoni and showed 100% identity with a reindeer-derived isolate previously reported from Finland. These findings provide the first confirmed record of R. andersoni in Norway and expand the known distribution of this climate-sensitive parasite, underscoring the need for targeted surveillance to clarify its occurrence, transmission dynamics, and potential health and management implications for semi-domesticated reindeer.